Semiconductor devices, such as magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Information is stored in the magnetic memory cells as an orientation of the magnetization of a free layer in the magnetic memory cell as compared to an orientation of the magnetization of a fixed (e.g., reference) layer in the magnetic memory cell. The free layer and the fixed layer, separated by a tunnel barrier, form a magnetic tunnel junction.
The magnetization of the free layer can be oriented parallel or anti-parallel relative to the fixed layer, representing either a logic “1” or a logic “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right. Driving a current through the magnetic tunnel junction can cause the magnetization of the free layer to switch due to spin transfer torque from left to right, and vice versa, to write information to the magnetic memory cell. See, for example, Worledge et al., “Spin torque switching of perpendicular Ta|CoFeB|MgO-based magnetic tunnel junctions,” Applied Physics Letters 98, 022501 (January 2011) (hereinafter “Worledge”), the contents of which are incorporated by reference as if fully set forth herein. As described in Worledge, with spin torque MRAM devices perpendicular magnetic anisotropy greatly reduces the switching voltage.
To achieve a reliable reading and writing on spin torque MRAM devices, a stable reference layer is a key prerequisite. The reference layer has to be rigid both under magnetic field and under the application of current. Therefore, an ideal reference layer has to own a large coercive field and a strong anisotropy energy density. Meanwhile, in order to operate the MRAM device under zero or small external magnetic field, the reference layer has to induce a minimal dipole field on the free layer. This means that the magnetic moment of the reference layer has to be largely internally canceled. For instance, multilayer structures based on cobalt (Co)/platinum (Pt) (or Co/palladium (Pd), Co/nickel (Ni), etc.) have been proposed for use as a reference layer in MRAM devices. However, the [Co/Pt]N multilayers are ferromagnetically aligned and as such the magnetic moments of the neighboring Co layers are parallel coupled. Thus, a dipole moment will build up and, without a large external offset field, this type of reference layer will undesirably exert a large dipole magnetic field onto the free layer.
To overcome this issue, it was proposed in Worledge to insert ruthenium (Ru) between the Co/Pt layers, i.e., resulting in a [Co/Pt]N/Ru/[Co/Pt]N structure. By inserting Ru, the magnetic moment of the [Co/Pt]N multilayer on top of the Ru will point in the opposite direction of the magnetic moment of the [Co/Pt)]N multilayer below the Ru, cancelling each other out, and thus solving the issue of a large dipole moment. However, there are some notable drawbacks to use of Ru. First, the perpendicular magnetic anisotropy is usually compromised because of the introduction of Ru. Second, Ru diffuses throughout the layers during high temperature anneals compromising function of the device. Moreover, reference layers made from Co/Pt usually have poor performance in thermal stability. The perpendicular anisotropy and the magnetoresistance of the magnetic tunnel junction will degrade after high temperature annealing because of the diffusion of Pt (or Ni, Pd) in those materials.
Therefore, a stable and low dipole field reference layer for perpendicular magnetic anisotropy spin torque MRAM devices would be desirable.